Glass-melting furnace

ABSTRACT

A continuous glass-melting tank furnace includes a melting compartment including a melting tank having a lower part, and a superstructure equipped with heaters for receiving and melting raw batch material. Additionally included is a separate refining compartment including a refining tank and a superstructure equipped with a further heater, the refining tank having a lower part and including a transverse sill which divides the refining tank into an upstream refining cell and a downstream refining cell, each of the upstream refining cell and the downstream refining cell having respective upstream ends and downstream ends, and the further heater being arranged to heat melt in the upstream refining cell for creating a spring zone located closer to the downstream end of the upstream refining cell and a circulation of melt in the upstream refining cell which feeds the spring zone. A throat allowing communication between the lower parts of the melting tank and the refining tank, and a conditioning tank for receiving melt from the refining tank are also included.

This is a division of application Ser. No. 07/188,553 filed Apr. 29,1988, now U.S. Pat. No. 4,929,266.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing glass in which rawmaterial is fed as a batch to a continuous glass-melting tank furnace.The method comprises melting the batch in a melting tank and passing themelt to a refining tank via a submerged throat, heating the melt in therefining tank to de-gas it, delivering molten refined glass to aconditioning tank and there bringing it to a desired workingtemperature. The invention includes glass manufactured by such a method,and it extends to a continuous glass-melting tank furnace having amelting compartment comprising a tank and superstructure equipped withheating means defining a melting zone for receiving and melting rawbatch material, a separate refining compartment also comprising a tankand superstructure equipped with heating means, means defining a throatallowing communication between the lower parts of the melting andrefining tanks, and a conditioning tank for receiving melt from therefining tank.

2. Description of the Related Art

In the manufacture of glass on an industrial scale, various problemsarise. Among these problems are that of economy regarding heating costs,and that of obtaining a properly refined, bubble-free glass.

It is of course well known that economies of scale are possible, andthat a melting furnace of any given size will be most economical whenrun at its designed production rate. In the remainder of thisspecification it will be assumed that any furnace referred to is beingrun at a given, most economical production rate.

It is well known that the reactions which take place between theconstituents of the raw batch during melting give rise to a considerableamount of surface foam on the melt, and bubbles of gas within the melt.It is also known that in order to refine the glass, that is to say, toensure the substantially no bubbles remain in the melt which is drawnoff for shaping, temperatures are required which are higher than thosewhich are in fact necessary for melting the glass.

Classical glass-melting furnaces have a single tank in which melting andrefining take place. Material in the tank is heated from above byburners, and the tank holds a molten mass which at the charging end ofthe tank is covered by unmelted or only partly melted batch material,and by foam. Somewhere close to the center of the tank there will be apoint, the "hot spot" where the melt has its highest temperature andthus least density. Accordingly there will be a "spring zone" of risingcurrents within the melt. At the walls of the tank, the melt will be atits coolest, and there will be falling currents there. As a result,there will be a return surface current flowing from the spring zone tothe charging end of the tank which tends to maintain unmelted batch andfoam in the upstream portion or melting zone of the tank so that suchbatch and foam cannot be drawn off at the downstream end of the refiningzone. Such currents will also tend to carry heat energy away to thewalls of the tank where it becomes dissipated, and it is not possible toexercise any degree of independent control of the temperatures of themelt in the melting and refining zones of the tank.

In an effort to obtain greater heat economy, proposals have been made todivide the furnace into separate melting and refining tanks. By workingin this way, it is possible to exercise a considerable degree ofindependence in control of the temperatures in the melting and refiningtanks. As a result, the melting tank can be run at lower temperaturesthan are required in classical furnaces with consequent savings inheating costs.

An example of such a plural-tank melting furnace is described in FrenchPatent Specification No. 2,550,523 (Saint-Gobain Vitrage SA). Accordingto the proposals of that specification, glass feeds from the bottom of amelting tank through a throat into the base of a separate refining tankwhich is shaped as a chimney up which the melt flows in a uniformascending current while being heated. The melt then passes directly to aconditioning tank where it is brought to a desired working temperature.In fact the principal source of heat both for melting and refining theglass is electric current, though optional burners over the refiningchimney are shown.

The cost savings which can be realized by using the previously proposedplural-tank melting furnaces are however attainable only at the expenseof a lowering of the homogeneity of the glass leaving the furnace. Thereis also an occasional tendency for the glass to be incompletelydegasified. The formation of the refining tank as a relatively deepchimney and the employment of submerged electrical heaters to maintain astrong ascending current of glass in this chimney as proposed in theabove mentioned French Patent Specification No. 2,550,523 would notavoid these disadvantages.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of manufacturingglass which facilitates the economical production of glass of a givencomposition and quality.

According to the present invention, there is provided a method ofmanufacturing glass in which raw material is fed as a batch to acontinuous glass-melting tank furnace, which method comprises meltingthe batch in a melting tank and passing the melt to a refining tank viaa submerged throat, heating the melt in the refining tank to de-gas it,delivering molten refined glass to a conditioning tank and therebringing it to a desired working temperature, characterized in that therefining tank is divided into upstream and downstream refining cells bya transverse sill, and the melt in the upstream refining cell is heatedto create a spring zone located towards the downstream end of that celland a circulation of melt in that cell which feeds the spring zone.

The adoption of the present invention facilitates the economicalproduction of glass of a given composition and quality.

By virtue of the presence of the spring zone in the upstream refiningcell, there will be a better defined circulation of the melt within thatcell. This promotes refining of the melt, and also, it promotes a goodmixing of the melt in that region. Furthermore, it is likely thatsurface return currents will be created to flow in the upstreamdirection over part of the area of the melt in the upstream refiningcell. Any such currents would act to constrain foam floating on the meltthere from flowing in the downstream direction, over the transverse silland towards the conditioning tank. Melt flowing in the downstreamdirection over the transverse sill will be close to the spring zone andthus close to the hottest part of the tank, and because of the relativeshallowness of the melt over the sill, any residual bubbles in the meltcan escape fairly easily there. Thus, for a given composition andquality of glass being produced, the method can be performed with therefining tank being run at a lower temperature than would otherwise berequired, and thus more economically.

Furthermore, because of the pattern of current in the melt in theupstream portion of the refining tank, a greater bubble population canbe tolerated in the melt feeding into the refining tank. Accordingly,the melting tank can also be run cooler for a given composition andquality of glass, thus affording further economies.

A further advantage of manufacturing glass by a method according to thepresent invention is that it facilitates switching over from theproduction of glass of one composition to glass of another. Because thefurnace is divided up into separate melting and refining tanks, andbecause the refining tank sole is provided with a sill, distinct currentcirculation patterns are set up in the melt. The result is that when thebatch composition is changed, for example from a batch for producingclear glass to one for producing colored glass, the change incomposition in the melt tends to take place much more rapidly than itwould otherwise, and the quantity of waste glass of an intermediatecomposition is reduced. It may be noted here that such waste glass of anintermediate composition is often difficult to make use of even ascullet for remelting. If such waste glass is to be used as cullet, thenecessary care must be taken to adjust the other ingredients of the rawbatch constantly depending on the varying composition of the cullet.

The shape of the volume occupied by the melt in the upstream refiningcell has an important influence on the currents in that cell. Inpreferred embodiments, the level of the surface of the melt is soregulated that the length of the upstream refining cell is greater thanthe mean depth of melt in that cell. The adoption of this feature isconducive to the formation of a continuous pattern of circulatingcurrents in the upstream part of that cell, and this further promotesrefining and homogenization of the melt in that region.

Advantageously, the mean length of the upstream refining cell is atleast equal to half of its means width, and preferably, the transversesill is spaced from the upstream end wall of the refining tank by adistance which is at least equal to the mean width of the upstreamrefining cell. When adopting one or both of these features, the anglesubtended by the upstream end wall of the refining tank at the springzone is less than it otherwise would be. As a result, any surface returncurrents flowing from the spring zone towards the upstream end wall ofthe refining tank make a more acute angle with the longitudinaldirection of the furnace and they may thus have an improved restrainingeffect on any foam on the melt in the refining tank and tend to pin itagainst the upstream end wall of the refining tank so that it cannotflow to the conditioning tank.

It is preferred that the level of the surface of the melt is soregulated that the mean depth of melt above the transverse sill is atmost two fifths of the mean depth of the melt in the downstream refiningcell. In operation, there is likely to be a return current of moltenglass which flows back from the downstream refining cell, over thetransverse sill, and into the upstream refining cell. This returncurrent, which may even flow from the conditioning tank, will consist ofglass which is cooler than that which forms a forward current flowingdownstream from the upstream refining cell. As a result, the forwardcurrent flowing over the transverse sill will tend to be confined to asurface layer which, by the adoption of this preferred feature, will berather less than two fifths of the depth of the melt in the downstreamrefining cell. Since the melt feeding that forward current must comefrom the rather close spring zone at the hottest part of the refiningtank, that forward current will itself be strongly heated, and strongheating of a rather thin surface layer is highly beneficial for refiningof the melt.

The furnace may be heated electrically using electrodes immersed in themelt, and/or by burners, the choice being a matter of convenience andeconomy. In preferred embodiments of the invention, the refining tank isheated at least in part by heaters which heat the melt most strongly ata location towards the downstream end of the upstream refining cell.This is a very simple way of creating a spring zone in the melt close tothe transverse sill, without unduly heating the wall structure whichseparates the melting and refining compartments, with consequentbenefits to the resistance of that wall structure to erosion by themelt. The adoption of this feature also promotes heating of the meltflowing over the transverse sill.

It is also preferred that there is a heater located to heat the meltabove said transverse sill. This promotes substantially completerefining of the melt.

Advantageously, the depth of the melt in at least a part of the meltingtank is less than the depth of the melt in at least a part of therefining tank. By adopting this feature, the melting tank can be madeshallower so that it will contain less melt, and as a result heatingeconomies can be effected. It will be appreciated that most if not allof the melt in the melting tank will be covered by unmelted batchmaterial or by foam. This tends to shield the sole of the melting tankfrom the heating effect of any burners in the melting compartment. Therefining tank on the other hand should contain no unmelted material, andany foam there should be substantially confined to its upstream end. Acertain depth of melt in the refining tank is therefore desirable, notonly for allowing room for a beneficial circulation of the melt, butalso for allowing a certain measure of shielding of the refining tanksole by the melt against the effect of burners over that tank, so as toreduce the tendency of the refining tank sole to be eroded by the melt.

Preferably, the melt flows from the melting tank into the refining tankvia a rising passageway. This is effective in preventing return currentsflowing upstream from the refining tank back into the melting tank, andis therefore beneficial for heat economy, and also for promoting a morerapid changeover between the manufacture of glasses of differentcompositions.

In some such embodiments, the melt is advantageously caused to flow fromthe melting tank into the refining tank through a throat located beneaththe level of the sole of the upstream refining cell. Dropping the levelof the throat in this way tends to give an increased cooling at thethroat: the sole and end walls of the throat may project from the baseof the tank furnace so that there will be increased heat radiation fromthe refractory parts making up the throat. As a result, the meltentering the refining tank will tend to be cooler, and it will thereforeenter the refining tank as a forward flowing bottom current which ismore viscous than the melt already in the refining tank. It will beapparent that the flow rates and the forces driving the forward andreturn currents in the refining tank upstream of the transverse sillmust be in balance. Accordingly, because of the viscosity differencesbetween the currents in the melt there, the cooler bottom current willtake up more space and will constrict the return current to a relativelyshallow surface layer. The surface return current will therefore becaused to flow faster. This is beneficial for stabilizing the currentcirculation and it promotes the confinement of any foam against the wallstructure separating the melting and refining compartments, andeffective refining of the melt.

Alternatively, or in addition the melt may with advantage be caused toflow over a second sill provided towards the upstream end of theupstream refining cell. Such a second sill can act as a barrier whichrestricts the volume of the space occupied by the surface returncurrent, and accordingly also has the effect of increasing its speed.Again, current stabilisation, foam retention and effective refining arepromoted. Care must be taken when adopting this feature however, becauseit has the consequence that the forward current flowing along the soleof the refining tank will be at an increased temperature. This increasein temperature should not be so high as to cause unacceptable erosion ofthe sole of the upstream refining cell.

Advantageously, the melt in the upstream refining cell is heated by atleast one immersed electrode. The use of such an electrode will have aneffect on the density of the melt in its immediate vicinity, and itaccordingly enables very fine control of the pattern of flow currents inthe melt. In particular, by locating such an electrode at or slightlyupstream of the spring zone, the location of the spring zone can bebetter defined or stabilized, thereby promoting a beneficial circulationof the melt for refining and mixing it.

In some preferred embodiments of the invention, gas is injected into themelt at the spring zone in the upstream refining cell. It may seemsomewhat contradictory to introduce gas into the melt in the refiningtank, but it should be kept in mind that the purpose of refining is toremove the rather small gas bubbles in the melt due to meltingreactions. Much larger gas bubbles can be introduced by injection. Itwill be appreciated that the forces causing bubbles in the melt to risedepend on the cube of the bubble radius while the forces hindering suchrise depend on the square of their radius. Such injected bubbles willhave the effect of stabilizing the position of the spring zone,constraining the rising currents there to flow in a more nearly verticaldirection and more quickly, and this promotes a stable pattern ofcirculating currents in the melt and thus refining of the melt. Such gasinjection is also beneficial in reducing the time required for changingthe composition of the glass being produced.

In embodiments of the invention in which the melt is heated by one ormore immersed electrodes and in which gas is injected as discussedabove, it is especially preferred that the melt in the upstream refiningcell is heated by at least one immersed electrode at a location closerto the upstream end of that cell than a location where gas is injectedinto the melt. The adoption of this preferred feature has been found topromote a particularly favorable and stable pattern of flow currentswithin the melt in the upstream refining cell.

Advantageously, the melt is caused to flow from the refining tank to theconditioning tank via a neck. This provides a constraint on flow betweenthe refining tank and the conditioning tank, in particular in reducingreturn currents from the conditioning tank to the refining tank, whichis beneficial for the pattern of current flow in the furnace. Also suchconstraint is of advantage if it is desired to change from theproduction of glass of one composition to glass of another composition:such changeover can be effected more quickly with a consequent saving inwaste glass of an intermediate composition.

Preferably, the melt is caused to flow from the refining tank to theconditioning tank beneath a floater provided at the downstream end ofthe refining tank. The presence of such a floater causes the meltentering the conditioning zone to do so from subsurface currents in therefining tank, and it provides an effective final safety barrier againstthe entry of surface foam into that conditioning zone.

In the most preferred embodiments of the invention, the maximumtemperature of the glass in the refining tank is kept higher than themaximum temperature of the glass in the melting tank. This promotes fueleconomy insofar as the melting tank is not heated to the hightemperatures required for refining the glass.

Advantageously, the maximum temperature of the glass in the refiningtank is maintained at a value which is at least 70° C. greater than themaximum temperature of the glass in the melting tank. This promotesrapid refining of the glass. In fact, the speed of refining is increasedby increasing the temperature in the refining tank, so for the mostrapid refining, the tank could be run at a temperature as hot as couldbe withstood by the refractory material of which it is formed. Howeverin order to limit heat losses from the refining tank, such temperaturedifferential is preferably not more than 300° C. It has been found that,when using any given furnace and for any given quality and compositionof glass, the maintenance of such a temperature differential gives thegreatest benefit in fuel economy.

The invention is applicable to the manufacture of many different typesof glass. It will be appreciated that the optimum temperatures to bemaintained in the melting and refining tanks will depend on the type ofglass being produced. For example borosilicate glasses will in generalrequire higher temperatures than soda-lime glasses to achieve a givenquality. However general statements for all types of glass can be madeby referring to the temperature at which the logarithm (to base 10) ofthe viscosity of the glass in Poises (10 P eqaul 1 pascal second) has agiven value, say N: this is denoted by the expression "theN/temperature". In this description references to the N temperature willbe followed by references in parenthesis to actual temperature valueswhich are the corresponding temperatures for soda-lime glass.

It is preferred that the maximum temperature in the refining tank ismaintained between the 2.08 temperature (1450° C.) and the 1.85temperature (1525° C.). Alternatively, or in addition, it is preferredthat the maximum temperature in the melting tank is maintained betweenthe 2.42 temperature (1350° C.) and the 2.16 temperature (1425° C.).Within those ranges, the maximum temperature required in the refiningtank is largely governed by the desired quality of the glass beingproduced, and the maximum temperature required in the melting tank isgoverned both by glass quality and by the presence or absence of meltingaccelerators such as sodium sulphate which may be included in the batch.Thus for example when melting glass for the production of float glass,it would be desirable to work towards the upper ends of the specifiedtemperature ranges, but for the manufacture of for example bottle glassit would be sufficient to work at the lower ends of those temperatureranges, especially if melting accelerators were to be included in thebatch material.

By way of comparison, it may be noted that the maximum temperature in aconventional furnace in which glass for the production of float glass ismelted and refined in a single tank is, for a particular batchcomposition, between the 1.85 temperature (1525° C.) and the 1.75temperature (1550° C.). The present invention can be used for theproduction of float glass of the same quality from the same batchcomposition while working within the temperatures ranges referred toabove. Accordingly, the maximum temperature in the refining zone can belower, and that in the melting zone can also be lower, than when using aconventional process, and this reduced requirement for high temperaturesleads to further economy in the use of fuel.

In preferred embodiments of the invention, substantially the wholesurface of the melt in the melting tank is covered by unmelted andpartially melted batch material. This ensures concentration of heat ontothe batch material to be melted, and substantially avoids the presenceof clear surface areas of the melt in the melting tank. If such areaswere present, there would be a direct path for radiation from the tanksuperstructure to the refractory material forming the sole of the tankand this could cause overheating of that material. Such overheatingwould lead to increased heat loss through the melting tank sole, andwould also shorten the useful working life of the refractory solematerial.

Advantageously, the plan area of the refining tank is at least as greatas that of the melting tank. The adoption of this feature has been foundto be particularly beneficial for the economical manufacture of wellrefined glass.

In some preferred embodiments of the invention, melt is fed from theconditioning tank to a float chamber. The use of a float chamber isparticularly advantageous for the manufacture of sheet glass of highquality. Alternatively or in addition, melt can be fed from theconditioning tank to a drawing machine. This is particularly appropriatefor the manufacture of sheet glass which is too thin to be madeconveniently by the float process.

The present invention includes glass manufactured by a method ashereinbefore defined.

The invention also extends to the furnace for the manufacture of glass.The invention provides a continuous glass-melting tank furnace having amelting compartment comprising a tank and superstructure equipped withheating means for receiving and melting raw batch material, a separaterefining compartment also comprising a tank and superstructure equippedwith heating means, means defining a throat allowing communicationbetween the lower parts of the melting and refining tanks, and aconditioning tank for receiving melt from the refining tank,characterised in that the refining tank is divided into upstream anddownstream refining cells by a transverse sill, and the heating means inthe refining compartment is arranged to heat melt in the upstreamrefining cell to create a spring zone located towards the downstream endof that cell and a circulation of melt in that cell which feeds thespring zone.

Such a furnace facilitates the economical production of glass of a givencomposition and quality, for example by a process as hereinbeforedefined. The furnace construction allows controlled circulation of meltcontained in the upstream cell of the refining tank which is beneficialfor refining the glass. Also, such a furnace is quite easy to build. Forexample in contrast to the furnace disclosed in French PatentSpecification No. 2,550,523 (Saint-Gobain Vitrage SA), substantially thewhole furnace comprising the melting tank, the refining tank and theconditioning zone can be constructed with its sole at the same or nearlythe same level. Because the furnace of French Patent Specification No.2,550,523 requires a vertical refining chimney, it is necessary that thesoles of the melting and conditioning zones be at very different levels,and this in turn involves substantial additional work in building thesupport structure for the conditioning zone (and any forming apparatusdownstream of the conditioning zone) which is not required for theconstruction of a tank furnace according to the present invention.

Preferably, the mean depth of the upstream cell of the refining tank isless than the length of that cell. This promotes the formation of acontinuous pattern of circulating currents in a melt in the upstreampart of that cell, and this further promotes refining and homogenizationof the melt in that region.

Advantageously, the mean length of the upstream cell of the refiningtank is at least equal to half of its mean width, and it is preferredthat the transverse sill is spaced from the upstream end wall of therefining tank by a distance which is at least equal to the mean width ofthe upstream refining cell. The adoption of one or both of thesefeatures has a beneficial effect on the pattern of current flow in themelt in that region of the refining tank, and it also allows room foradequate heating of that melt without subjecting the wall structureseparating the melting and refining compartments to such excessive heatas would unnecessarily shorten it working life due to erosion.

Preferably, the mean height of the transverse sill above the sole of thedownstream cell of the refining tank is at least three fifths of themean depth of that downstream cell. A sill of such height is beneficialfor stabilising current flow patterns and for promoting good refining ofmelt flowing over it.

Advantageously, the refining compartment is provided with heaters which,considered as a group, are located closer to said transverse sill thanto the upstream end of that compartment. This is a very simple way ofproviding the heating means required. Of course such heaters may besupplemented with other heating means if required, for example withheating electrodes which project into the refining tank.

Preferably, there is a heater located to heat material flowing above thetransverse sill. This is beneficial for ensuring heating and refining ofa forward flowing surface current of the melt which flows across thatsill.

Advantageously, the sole of at least a part of the melting tank is at ahigher level than the sole of at least a part of the refining tank. Thisallows the use of a melting compartment of smaller capacity which cangive useful savings in fuel consumption, while at the same time allowinga measure of protection to the sole of the upstream cell of the refiningtank against overheating and erosion, due to the depth of melt which isabove it in use.

Preferably, the throat communicates with the upstream refining cell viaa rising passageway. This is effective in preventing return currentsflowing upstream from the refining cell back into the melting tank, andis therefore beneficial for heat economy, and also for promoting a morerapid changeover from the manufacture of glass of one composition toglass of another.

In some such embodiments, it is preferred that the throat is beneath thelevel of the sole of the refining tank. It is quite simple to drop thelevel of the sole of the furnace over the rather small area necessary todefine such a throat. In addition to having a beneficial effect on theflow pattern of the melt between the shadow wall and the transverse sillas has previously been adverted to, dropping the level of the throat inthis way allows the refractory defining the throat to be maintained at alower temperature, thus making that refractory less liable to erosion.

Alternatively, or in addition, a second sill may be provided towards theupstream end of the refining tank. Such a second sill is very easy toinstall, and can have a similar beneficial effect on the flow pattern ofthe melt. This sill can also act to shade the region of the throat fromthe heaters in the refining zone, thus again prolonging the working lifeof the refractory defining the throat. It will be appreciated that thatsecond sill will itself be exposed to quite strong heating in operationof the furnace, so it should be made of a rather high grade refractorymaterial. Also, the use of such a sill can have the effect of increasingthe temperature of the currents flowing along the bottom of the upstreamrefining cell between the two sills, and consideration shouldaccordingly be given as to whether it is necessary to make that portionof the sole of a higher grade refractory than would otherwise be done.

Advantageously, at least one heating electrode is provided for immersionin the melt in the upstream refining cell. The use of such an electrodeenables very fine control of the pattern of flow currents in the melt.In particular, by locating such an electrode at or slightly upstream ofthe spring zone, the location of the spring zone can be better definedor stabilized thereby promoting a beneficial circulation of the melt forrefining and mixing it.

In some preferred embodiments of the invention, a means is provided forinjecting gas into the refining tank at the spring zone. This stabilizesthe spring zone and has a beneficial effect on the current circulationpattern in the melt.

In embodiments of the invention in which the melt is heated by one ormore immersed electrodes and in which gas is injected as discussedabove, it is especially preferred that at least one heating electrode isprovided at a location closer to the upstream end of that cell than thelocation of the gas injection means. The adoption of this preferredfeature has been found to promote a particularly favorable and stablepattern of flow currents within the melt in the upstream refining cell.

The refining tank is preferably connected to the conditioning tank via aneck. Such a neck is very simple to construct, and its use has afavorable effect on the flow pattern in the melt, particularly inreducing return currents, and on the speed with which a change can bemade from the production of glass of one composition to glass ofanother.

Advantageously, a floater is provided at the downstream end of therefining tank. Such a floater can prevent any material floating on topof the melt from flowing further downstream. If such a floater isprovided located in a neck between the refining tank and theconditioning tank, it can be made shorter than if it is located in therefining tank itself.

Advantageously, the plan area of the refining tank is at least as greatas that of the melting tank. The adoption of this feature has been foundto be particularly beneficial for the economical manufacture of wellrefined glass.

The invention is particularly suitable for the production of a highquality melt which is suitable for forming into sheets, for example bythe float process. In preferred embodiments, therefore, the conditioningtank is connected for feeding molten glass to a float chamber.

Alternatively, or in addition, it is preferred that the conditioningtank is connected for feeding molten glass to a drawing machine. Suchembodiments are particularly suitable for the production of sheet glasswhich is thinner than can conveniently be made by the float process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be further described with reference tothe accompanying diagrammatic drawings in which:

FIGS. 1 and 2 are respectively sectional plan and side views of acontinuous glass-melting tank furnace according to the invention, whichcomprises a melting compartment, a refining compartment and aconditioning tank.

FIG. 3 is an enlarged cross-sectional sideview of the refiningcompartment of the tank furnace of FIG. 1, and

FIG. 4 is a cross-sectional side view of the refining compartment of afirst alternative embodiment of tank furnace.

FIGS. 5 and 6 are respectively cross-sectional plan and side views of asecond alternative embodiment of tank furnace,

FIG. 7 is a cross-sectional side view of a third alternative embodimentof tank furnace,

FIGS. 8 and 9 are respectively cross-sectional plan and side views of afourth alternative embodiment of tank furnace,

FIGS. 10 and 11 are respectively cross-sectional plan and side views ofa fifth alternative embodiment of tank furnace,

FIGS. 12 and 13 are respectively cross-sectional plan and side views ofa sixth alternative embodiment of tank furnace, and

FIG. 14 is a cross-sectional side view a seventh alternative embodimentof tank furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, a continuous glass-melting tank furnace comprises amelting compartment 1 including a tank 2 which is in melt flowcommunication with a tank 3 of a refining compartment 4 via a submergedthroat 5 beneath a wall structure 6 which constitutes the downstream endwall of the melting tank 2 and the upstream end wall of the refiningtank 3. On the sole of the refining tank 3 is located a transverse sill7 which divides the refining tank 3 into upstream and downstreamrefining cells 8 and 9. In the embodiment illustrated, the length of theupstream refining cell 8 is greater than its depth, and that length isalso greater than the width of the upstream refining cell 8. At thedownstream end of the refining tank 3 is provided a neck 10 givingcommunication with a conditioning tank 11 whence molten glass may bedrawn off and fed to a glass shaping apparatus not shown. Such a glassshaping apparatus may, and preferably does, comprise a float chamberand/or a flat glass drawing machine. The outlet of the conditioning tank11 illustrated is in fact designed for feeding to a float chamber. Theshaping apparatus may alternatively, or in addition, take the form ofone or more rolling machines for the production of figured glass, ormolding machines for the production of glass bottles or other hollowware. It will however be appreciated that quality requirements forfigured glass and hollow ware are not usually so high as those for sheetglass.

A second optional sill 12 is provided a short distance downstream of thethroat 5 to define a rising passageway 13 through which the melt entersthe refining tank 3. For this purpose, the top of that second sill 12 islocated at a level which is higher than the top of the throat 5.

The melt surface level is represented in FIG. 2 by the line 14. Afloater 15 is positioned at the downstream end of the refiningcompartment 4 in the entrance of the neck 10.

In FIGS. 3 and 4, those parts which are also shown in FIGS. 1 or 2 areallotted the same reference numerals. FIGS. 3 and 4 also show how thewall structure 6 separates the atmospheres contained by superstructures16 and 17 respectively of the melting and refining compartments 1 and 4.Also shown is the downstream end burner 18 for each melting compartment1, and three transverse burners 19, 20, 21 in each refining compartment4, of which the downstream one 21 is located over the transverse sill 7.These burners 19, 20, 21 are located and adjusted to maintain a springzone represented by arrow 22 in the upstream cell 8 of the refining tank3 which is upstream of the transverse sill 7, but closer to that sillthan to the wall structure 6.

In the embodiment shown in FIGS. 1, 2 and 3, the sole 23 of the meltingtank 1 is at the same level as the sole 24 of the upstream cell 8 of therefining tank 3, upstream of the transverse sill 7, and this level isslightly higher, for example about 0.3 m, than the level of the sole 25of the refining tank 3 downstream of that transverse sill 7 whichcontinues to form the sole of the neck 10 and the conditioning zone 11.

In operation of the embodiment shown in FIG. 3, there will be a forwardflow of melt through the throat 5 and up the rising passage 13. Becauseof the configuration of this rising passage, there can be substantiallyno return flow from the refining tank 3 to the melting tank 2, providedthat the refining tank is maintained hotter than the melting tank sothat the melt in the refining tank is less dense than that entering it.Melt flowing up the rising passage 13 will flow over the second sill 12as a sub-surface current because it is cooler than the melt which haspreviously been exposed to the burners 19 to 21, and it will thereforealso form a falling current on the downstream side of that second sill12 feeding a forward flow of melt in the upstream refining cell 8between the two sills, leading towards the spring zone 22. Because themelt there is at its hottest and least dense, it will form a risingcurrent which will flow outwards in all directions across the surface ofthe melt. Part of that surface flow will be constituted by returnsurface currents flowing back towards the wall structure 6. The anglesubtended by the wall structure 6 at the spring zone 22 will clearly besmaller as the spacing between them increases. As a result, the surfacereturn currents directed back towards the wall structure in theembodiment illustrated can have a sufficient component in thelongitudinal direction of the furnace to confine against the wallstructure any bubbles which rise to the surface of the melt in theupstream refining cell 8 upstream of the spring zone. Surface returncurrents flowing to the wall structure will be cooled slightly bycontact with that wall structure and/or by contact with melt enteringthe upstream refining cell from the melting tank, and they willtherefore descend to join freshly introduced melt and circulate backdown the second sill 12 and along the sole 24 to the spring zone 22.Surface currents flowing downstream from the spring zone 22 will flowover the transverse sill 7 into the downstream cell 9 of the refiningcompartment 4 and thence through the neck 10 to the conditioning tank11. In the conditioning tank 11, (not shown in FIG. 3,) melt coming intocontact with the side and end walls will also be cooled to form sinkingcurrents, and these will feed bottom return currents flowing along thesole 25. Flow of these currents back into the refining tank 3 will berestricted by the presence of the neck 10, but nevertheless, there willbe some melt in these currents which will flow to form a rising currentat the downstream side of the transverse sill 7 and this will flow upover that sill and descend to feed the base of the spring zone 22 fromthe downstream end. The presence of this over sill return current forcesa very shallow forward surface current over the sill so that melt inthat forward current is well exposed to heat from the downstream burner21 over the sill 7. This system of currents promotes good mixing andrefining of the melt in the refining tank.

In the absence of the optional second sill 12, melt flowing through thethroat 5 will tend to flow as a forward bottom current directly to thebase of the spring zone 22. Again return surface currents will begenerated and maintained, but since these return currents will not beimpeded by the presence of the second sill, they can descend to the baseof the wall structure and then join the forward bottom current feedingthe base of the spring zone. In this case, there might be a slightreturn current through the throat.

With the presence of the second sill 12, the sole 24 of the upstreamrefining cell 8 will tend to be hotter than when that sill is notpresent. This will of course lead to an increased rate of erosion of thesole 24, even to such an extend as to shorten its working life to anunacceptable degree. It may not always be possible to compensate forthis adequately by reducing the heating of the upstream refining cell 8having regard to the temperatures which are necessary to effect adequatedegassing of the melt. One way of compensating would be to make the sole24 of a higher grade refractory than would be required if the secondsill were not present. Another way of compensating would be to drop thelevel of the sole 24 of the upstream refining cell 8, for example to thelevel of the sole 25 of the downstream refining cell 9. The additionaldepth of melt in the upstream refining cell 8 would then have anincreased shielding effect on the sole 24 against radiant heat from theburners 19 to 21.

In the embodiment of FIG. 4, the sole 23 of the melting tank 2 slopesdown at its downstream end as shown at 26 to form a sunken throat 5,below the level of the sole 24 of the upstream refining cell 8. Thatthroat sole 27 is connected to the upstream refining cell sole 24 by awall 28 which, with the wall structure 6 defines a rising passage 13 forthe melt to enter the refining tank from the melting tank. A sill 29 isprovided in the melting tank 2 at the junction between the horizontaland sloping portions 23 and 26 of the tank sole to encourage a risingflow of melt in the melting tank 2 and thus impede any direct forwardbottom flow of partially melted material from the melting tank into thethroat. In this embodiment, the flow pattern downstream of the immediatevicinity of the throat is very similar to that of the FIG. 3 embodimentwithout the optional second sill. It will be noted though that therewill be very little, if any, possibility of glass forming a returncurrent flowing back through the throat from the refining tank. It is tobe noted that such a second sill could be provided in the embodimentshown in FIG. 4 if desired, for example above the throat end wall 28.

In the FIG. 4 embodiment, the soles 24, 25 of the upstream anddownstream refining cells 8, 9 are at the same level, a level which islower, for example 30 cm lower, than the level of the horizontal soleportion 23 of the melting tank.

A specific embodiment of continuous glass-melting tank furnace designedin accordance with FIGS. 1 to 3 for the production of glass at a rate of50 tonnes per day has the following dimensions.

    ______________________________________                                        Width of melting tank 2                                                                              4.0 m                                                  Width of throat 5      0.7 m                                                  Width of refining tank 3                                                                             4.0 m                                                  Width of neck 10       1.2 m                                                  Width of conditioning tank 11                                                                        3.6 m                                                  Depth of melting tank 2                                                                              0.9 m                                                  Height of throat 5     0.3 m                                                  Depth of upstream refining cell 8                                                                    0.9 m                                                  Depth of downstream refining cell 9                                                                  1.2 m                                                  Depth of neck 10       1.2 m                                                  Depth of conditioning tank 11                                                                        1.2 m                                                  Depth of melt above transverse sill 7                                                                0.3 m                                                  Depth of melt above second sill 12                                                                   0.3 m                                                  Length of melting tank 2                                                                             4.5 m                                                  Length of throat 5     1.2 m                                                  Length occupied by passageway 13                                                                     0.6 m                                                  Length occupied by transverse sill 7                                                                 0.6 m                                                  Length between sills of upstream cell 8                                                              3.5 m                                                  Length occupied by second sill 12                                                                    0.6 m                                                  Length of downstream refining cell 9                                                                 4.0 m                                                  Length of neck 10      3.0 m                                                  Length of conditioning tank 11                                                                       6.0 m                                                  ______________________________________                                    

For the production of highly refined soda-lime glass of ordinarycomposition, such a furnace may be run with a maximum melt temperaturein the melting tank of about 1375° C. (the 2.33 temperature) while themaximum temperature of the melt in the refining tank is about 1475° C.(the 2.0 temperature).

In the embodiment shown in FIGS. 5 and 6, the melting compartment 1 isof the end-fired or horseshoe-flame type in which burner ports such as30 are provided in the charging end wall 31. A plurality of electrodes32 are immersed in the melt in the melting tank 2 to providesupplementary heat energy for melting the batch. The sole 23 of themelting tank 2 and the sole 24 of the upstream refining cell 8 are onthe same level so the melt enters that refining cell through a straightthroat 5. The sole 25 of the downstream refining cell 9, the neck 10 andthe conditioning tank 11 are also at that same level.

The refining compartment 4 is cross-fired by using three burner ports19, 20, 21 at each side. The downstream burner port 21 shown is locatedabove transverse sill 7 separating the upstream and downstream refiningcells 8 and 9. Additional heat energy is supplied to the upstreamrefining cell 8 using booster electrodes 33 projecting upwardly throughthe sole 24 of that cell, of which one electrode is locatedsubstantially in the center of the cell 8 and two are located towardsthe upstream end wall structure 6 of the refining compartment. The useof such booster electrodes 33 in the upstream refining cell 8 isbeneficial for promoting a desirable and stable flow pattern ofconvection currents in the melt in that cell.

The length of the upstream refining cell 8, that is the distance betweenthe transverse sill 7 and the upstream end wall 6, is greater than itswidth, and its width is in turn greater than the depth of melt in thatcell. The melting tank 2 and the refining tank 3 have the same width.The depth of melt above the transverse sill 7 is about one quarter ofthe total depth of melt in the tank furnace.

Refined melt leaving the downstream refining cell 9 passes beneathfloater 15 to enter the neck 10 and thence flows into the conditioningtank 11 to the outlet end of the furnace, here shown as a pouring spout34 for supplying molten glass to a rolling machine or float chamber (notshown).

A specific embodiment of continuous glass-melting tank furnace designedin accordance with FIGS. 5 and 6 for the production of glass at a rateof 250 tonnes per day has a melting tank 2 which is 89 m² (8.5 m×10.5 m)in plan area, a refining tank 3 which is 148 m² (8.5 m×17.4 m) in planarea, and a conditioning tank 11 which is 120 m² in plan area.

In the furnace of FIG. 7, the melting compartment 1 is cross-fired, andelectrodes 32 project up through the sole 23 to provide supplementaryenergy for melting the batch. The level of the melting tank sole 23 isdropped at its downstream end so that the throat 5 is beneath the levelof the melting tank sole. The sole 24 of the upstream refining cell 8 isat the level of the sole of the throat as is the sole 25 of thedownstream refining cell and the sole of the neck 10 and conditioningtank 11.

The refining compartment 4 of the furnace of FIG. 7 is broadly similardesign to that shown in FIGS. 5 and 6, except for the arrangement ofbooster electrodes 33 in the upstream refining cell. In FIG. 7, there isa row of four vertical electrodes 33 located closer to the sill 7 thanto the upstream end wall 6. The electrodes 33 may for example be locatedsubstantially along the neutral line of the cell 8, that is, thetransverse line passing through the spring zone (as compared to 22 inFIGS. 3 and 4). The use of such electrodes promotes upward flow of meltat the spring zone and gives a better definition of, or redefines, thelocation of that spring zone, thus promoting good mixing and refining ofthe melt.

On leaving the refining tank 3, the melt enters the neck 10 passingbeneath a bridgewall 35 which is clear of the surface of the melt, andthen passes to the conditioning tank 11 whence it may be fed to anydesired glass shaping apparatus.

The length of the upstream refining cell 8, that is the distance betweenthe transverse sill 7 and the upstream end wall 6, is greater than itswidth, and its width is in turn greater than the depth of melt in thatcell. The melting tank 2 and the refining tank 3 have the same width.The depth of melt above the transverse sill 7 is about two-fifths of thetotal depth of melt in the upstream refining cell 8.

A specific embodiment of continuous glass-melting tank furnace designedin accordance with FIG. 7 for the production of glass at a rate of 500tonnes per day has a melting tank 2 which is 141 m² (10 m×14.1 m) inplan area, a refining tank 3 which is 234 m² (10 m×23.4 m) in plan area,and a conditioning tank 11 which is 160 m² in plan area.

In the embodiment of FIGS. 8 and 9, the design of melting compartment 1is substantially as described with reference to FIGS. 1 and 2. The soleof the entire furnace is at the same level and the melt enters therefining compartment 4 through a straight throat 5.

The refining compartment 4 is of broadly similar design to thatdescribed with reference to FIGS. 5 and 6, the main differences beingthe arrangement of booster electrodes 33 and the provision of gasinjectors 36 in the upstream refining cell 8. Along the neutral line ofthat cell, a row of three gas injectors 36 projects upwardly through thesole 24. The central injector 36 is located to define the spring zone.Vertically spaced pairs 33a, 33b of booster electrodes project into themelt in refining cell 8 through its side walls. At each side of therefining cell, one pair 33a of booster electrodes is located spacedslightly upstream of the neutral line, and the other pair 33b is locatedspaced slightly downstream of that line. This arrangement of gasinjectors and booster electrodes is highly beneficial for obtaining awell-defined spring zone and a stable flow pattern in the melt for goodmixing and refining.

In a variant, the downstream pairs of booster electrodes 33b areomitted, and in another variant, additional upstream pairs of boosterelectrodes 33 are provided close to the indicated positions 33a. Thesearrangements are also highly beneficial for achieving good refining andmixing of the melt.

On leaving the refining tank 3, the melt enters a neck 10 which isnarrower than the necks 10 of previously described embodiments.Accordingly, no floater 15 or bridgewall 35 is provided at the entranceto the neck 10 in this embodiment. From the neck 10, the melt passesinto a conditioning tank 11 having twin outlets for feeding two glassshaping machines, for example drawing machines.

A specific embodiment of continuous glass-melting tank furnace designedin accordance with FIGS. 8 and 9 for the production of glass at a rateof 100 tonnes per day has a melting tank 2 which is 36 m² (6 m×6 m) inplan area, and a refining tank 3 which is 59 m² (6 m×9.8 m) in planarea.

FIGS. 10 and 11 illustrate an embodiment of a continuous glass-meltingtank furnace which is particularly suitable for the manufacture of glassat rather high production rates, for example 600 tonnes per day. Theentire furnace sole is at one level. The melting compartment 1 is ofsimilar design to that described with reference to FIGS. 1 and 2, andthe melt passes from the melting tank 2 into the refining tank 3 via astraight throat 5 which is wider than the throats 5 of previouslydescribed embodiments. The refining tank 3 is wider than the meltingtank 2.

The refining compartment 4 is cross-fired, and because of its highdesign capacity it is provided with four burner ports at each side. Thedownstream burner port 21 is located to heat melt downsteam of thetransverse sill 7 as well as melt flowing over that sill. The sill 7occupies some two-thirds of the total depth of the melt, and it islocated at a distance from the upstream end wall 6 of the refining tankwhich is about twice the depth of the melt, and approximatelyfive-sixths of the width of the refining tank.

A row of four gas injectors 36 is arranged along the neutral line of theupstream refining cell 8. A staggered transverse row of three boosterelectrodes 33 projects upwardly through the sole of that cell at alocation which is close to but upstream of the neutral line. A secondrow of booster electrodes 33c is preferably provided upstream of thefirst. If desired, such a second row of booster electrodes could belocated downstream of the neutral line.

A specific embodiment of a continuous glass-melting tank furnacedesigned in accordance with FIGS. 10 and 11 for the production of glassat a rate of 600 tonnes per day has a melting tank 2 which is 150 m² inplan area, a refining tank 3 which is also 150 m² in plan area, and aconditioning tank 11 which is 160 m² in plan area.

FIGS. 12 and 13 illustrate a continuous glass-melting tank furnace inaccordance with this invention.

In the melting compartment 1, batch material is melted by continuouslyoperating side burners 118 whose flames are constrained to lick thesurface of the material in the tank by virtue of a lowered portion 116of the melting tank superstructure. The fuel fed to the burners may beoil or gas. Flames and fumes are then drawn up through chimney 117.

The flow of melt from the melting tank 2 to the refining tank 3 iscontrolled by a sill 29 in the melting tank and a dropped narrow throat5 as described with reference to FIG. 4. The sole 23 of the melting tankis at the same level as the soles of the other compartments of thefurnace.

In the refining compartment, continuously operating side burners 119,120, 121 are provided at each side, and fumes and flames are drawn offfrom the refining compartment through a chimney 122. It is convenient touse gas burners in the refining compartment. The upstream end wall 6 ofthe refining compartment 4 is oblique. The transverse sill 7 is locatedso that the mean length of the upstream refining cell is greater thanits width. The width of that cell is in turn greater than its depth. Thesill occupies some four-fifths of the depth of the melt.

A transverse row of three booster electrodes 33 projects upwardlythrough the sole of the cell 8 at the neutral line. A second row ofbooster electrodes may be provided upstream of the first if desired.

Molten refined glass leaving the refining tank 3 passes through the neck10, into a conditioning tank 11 and thence directly into the drawingtank 123 of a horizontal glass drawing machine.

A specific embodiment of a continuous glass-melting tank furnacedesigned in accordance with FIGS. 12 and 13 for the production of glassat a rate of 50 tonnes per day has a melting tank 2 which is 20 m² (4m×5 m) in plan area, and a refining tank 3 which is 33 (4 m×8.3 m) m² inplan area.

FIG. 14 illustrates a further embodiment of a furnace for the continuousproduction of molten glass. In FIG. 14, the melting compartment is ofthe cupola type, in which melting is effected by means of a plurality ofvertical electrodes 124 leading through the sole 23 of the melting tank2 to provide heat energy for melting batch material 125 which isuniformly spread over the surface of the molten material in the tank 2.The melting tank 2 communicates with the refining tank 3 via a droppedthroat 5 (compare FIG. 13, through no sill is provided in the meltingtank). The design of the refining compartment 4, the neck 10 andconditioning tank is the same as that of the embodiment shown in FIGS.12 and 13, though the outlet end of the furnace is shown as beingprovided with a pouring spout 34 for feeding a float chamber or castingmachine.

We claim:
 1. A continuous glass-melting tank furnace, comprising:amelting compartment including a melting tank having a lower part, and asuperstructure equipped with heating means for receiving and melting rawbatch material; a separate refining compartment including a refiningtank having an upstream end and a downstream end and a superstructureequipped with a further heating means, the refining tank having a lowerpart and including a transverse sill which divides the refining tankinto an upstream refining cell and a downstream refining cell, each ofthe upstream refining cell and the downstream refining cell havingrespective upstream ends and downstream ends, and the further heatingmeans being arranged to heat melt in the upstream refining cell forcreating a spring zone located closer to the downstream end of theupstream refining cell and a circulation of melt in the upstreamrefining cell which feeds the spring zone; means defining a throatallowing communication between the lower parts of the melting tank andthe upstream end of the refining tank; and a conditioning tank forreceiving melt from the downstream end of the refining tank.
 2. Thecontinuous glass-melting tank furnace according to claim 1, wherein theupstream cell of the refining tank has a length and a mean depth, andwherein the mean depth is less than the length.
 3. The continuousglass-melting tank furnace according to claim 1, wherein the upstreamcell of the refining tank has a length and a means width, and whereinthe means length is at least equal to half of the mean width.
 4. Thecontinuous glass-melting tank furnace according to claim 3, wherein therefining tank includes an upstream end wall and wherein said transversesill is spaced from the upstream end wall of the refining tank by adistance which is at least equal to the mean width of the upstreamrefining cell.
 5. The continuous glass-melting tank furnace according toclaim 1, wherein the transverse sill has a mean height, and wherein thedownstream cell includes a sole and has a mean depth so that the meanheight of the transverse sill above the sole of the downstream cell ofthe refining tank is at least three fifths of the mean depth of thedownstream cell.
 6. The continuous glass-melting tank furnace accordingto claim 1, wherein the refining compartment superstructure includesheaters which, considered as a group, are located closer to saidtransverse sill than to the upstream end of the refining compartment. 7.The continuous glass-melting tank furnace according to claim 6, whereinone of said heaters is located to heat material flowing above saidtransverse sill.
 8. The continuous glass-melting tank furnace accordingto claim 1, wherein the melting tank includes a sole, wherein therefining tank includes a sole, and wherein the sole of at least a partof the melting tank is at a higher level than the sole of at least apart of the refining tank.
 9. The continuous glass-melting tank furnaceaccording to claim 1, further comprising means defining a risingpassageway, wherein the throat means communicates with the upstreamrefining cell via the rising passageway means.
 10. The continuousglass-melting tank furnace according to claim 9, wherein the upstreamrefining cell includes a sole and wherein the throat means is locatedbeneath the level of the sole of the upstream refining cell.
 11. Thecontinuous glass-melting tank furnace according to claim 9, wherein saidrising passageway means comprises a second sill provided towards theupstream end of the upstream refining cell.
 12. The continuousglass-melting tank furnace according to claim 1, further comprising atleast one heating electrode provided for immersion in the melt in theupstream refining cell.
 13. The continuous glass-melting tank furnaceaccording to claim 12, further comprising means for injecting gas intothe refining tank at the spring zone.
 14. The continuous glass-meltingtank furnace according to claim 13, wherein at least one said heatingelectrode is provided at a location closer to the upstream end of theupstream refining cell than the location of the gas injection means. 15.The continuous glass-melting tank furnace according to claim 1, furthercomprising a means defining a neck which connects the down stream end ofthe refining tank to said conditioning tank.
 16. The continuousglass-melting tank furnace according to claim 1, further comprising afloater provided at the downstream end of the refining tank.
 17. Thecontinuous glass-melting tank furnace according to claim 1, wherein therefining tank has a plan area, wherein the melting tank has a plan area,and wherein the plan area of the refining tank is at least as great asthe plan area of the melting tank.
 18. The continuous glass-melting tankfurnace according to claim 1, further comprising a pouring tankconnected to said conditioning tank for feeding molten glass to a floatchamber.
 19. The continuous glass-melting tank furnace according toclaim 1, further comprising a drawing tank connected to saidconditioning tank for feeding molten glass to a horizontal glass drawingmachine.